1. Technical Field of the Invention
This invention relates generally to wireless communications and more particularly to adjusting transmit power to conserve power and/or reduce interference.
2. Description of Related Art
As is known, a wireless communication from one wireless communication device to another in a wireless communication system can take many forms depending on the standard to which the system is compliant and operational conditions within the system. For example, the wireless communication system may be based on an IEEE 802.11 standard, which provides multiple data rates and modulation schemes for wireless communications. For instance, IEEE 802,11a standard provides data rates 6 Mega Bits Per Second (Mbps), 9 Mbps, 12 Mbps, 18 Mbps, 24 Mbps, 36 Mbps, 48 Mbps, and 54 Mbps and modulation schemes of Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), 16 QAM (Quadrature Amplitude Modulation), and 64 QAM.
As is also known, as the data rates increase, the signal to noise ratio (SNR) of a wireless communication must increase as well. This occurs because the higher data rate communications transmit more data per time interval than lower data rate communications and have smaller phase-amplitude differentiation between data values than the lower data rate communications. As such, for the receiver to accurately recapture the transmitted data for high data rate communications, the information details of the signal carrying the data must be readily discernable from noise.
To further enhance a receiver's ability to accurately recapture transmitted data, the transmitter may further encode the data using a coding rate. For instance, the IEEE 802.11a standard provides coding rates of ½, ⅔, and ¾, which indicates the ratio of uncoded bits to encoded bits. For example, rate ½ indicates that for every one bit entering the encoder, the encoder produces a two bit encoded output. In certain instances, lower coding rates (e.g., rate ½) are used for higher data rates and higher coding rates (e.g., rate ¾) are used for lower data rates. As such, the number of bits transmitted is not a linear relationship with the data rate.
As is further known, when a wireless communication is established, the transmitter and/or the receiver determine the data rate based, in part, on the signal-to-noise ratio. Accordingly, the transmitter and/or receiver select the highest data rate that can be supported by the channel, thereby using the least amount of bandwidth of the channel to fulfill the communication.
To achieve acceptable SNR for wireless communications, the transmitter typically transmits at a set power level, which is near the maximum power capabilities of the transmitter. Note that transmitters compliant with IEEE 802.11h utilize transmission power control (TPC) to limit the transmit power to a minimum level needed to reach the furthest receiver of the communication. In addition, transmitters compliant with the IEEE 802.11h standard use dynamic frequency selection (DFS) to select a channel that minimizes interference with other systems.
To achieve an acceptable signal-to-noise ratio for wireless communications, the transmitter typically transmits at a set power level, which is near the maximum power capabilities of the transmitter. Note that transmitters compliant with IEEE 802.11 h utilize transmission power control (TPC) to limit the transmit power to a minimum level needed to reach the furthest receiver of the communication. In addition, transmitters compliant with the IEEE 802.11 h standard use dynamic frequency selection (DFS) to select a channel that minimizes interference with other systems.
Based on the foregoing, a transmitter transmits data at a power level to reliably convey the data to a receiver, where the data is at the highest data rate, using the least amount of bandwidth of the channel, that can be supported by the channel. Accordingly, a peak power level is used while the data is being transmitter. While this provides an efficient use of the channel to support one or more wireless communications, the peak power levels may be at such levels as to interfere with communications on adjacent channels, produce a higher average power than desired due to non linearity of a power amplifier operating close to its compression point, and/or, for multiple communications, may push the transmit power capabilities of the transmitter.
Therefore, a need exists for a method and apparatus of adjusting transmit power of a wireless communication device to control the transmit power levels to reduce power consumption, to reduce peak and/or average power levels, and/or to reduce adjacent channel interference.